Bending is a crucial operation in the sheet forming of Mg alloys for automotive and aerospace applications.In this work,three kinds of sheets from an AZ31 rolled plate,sheared at 0°(S0),45°(S45),and 90°...Bending is a crucial operation in the sheet forming of Mg alloys for automotive and aerospace applications.In this work,three kinds of sheets from an AZ31 rolled plate,sheared at 0°(S0),45°(S45),and 90°(S90)to the rolling direction,were subjected to three-point bending tests.In-situ digital image correlation(DIC)system was employed to capture the strain fields.Electron backscatter diffraction(EBSD)was used to examine the through-thickness microstructures.A crystal plasticity finite element method(CPFEM)incorporating twinning and slip mechanisms was developed to simulate the bending deformation.The texture effect on the neutral layer shift and twinning behaviors was systematically investigated in terms of both experiments and simulations.The results show that CPFEM effectively simulated the texturedependent shifting behaviors of neutral layer and the associated twinning behaviors.Particularly,the spatial distributions of neutral layer across the entire bent plates were captured by both DIC and CPFEM.Additionally,some unusual twinning behaviors were analyzed in depth,such as the{10-12}-{10-12}secondary twins in S90,localized twin bands in S0,and the twin traces difference in S45.These findings reveal a close relationship between the neutral layer shift and twinning activity induced by the initial texture and strain levels.This work provides valuable insights into the bending deformation mechanisms of Mg alloys and has important implications for improving their formability and controlling springback.展开更多
The effects of grain size on the shift of neutral layer of AZ31 magnesium alloy sheets with different grain sizes ranging from 12.1 to 34.7μm were investigated by the 90° V-bending tests at 150 °C. The resu...The effects of grain size on the shift of neutral layer of AZ31 magnesium alloy sheets with different grain sizes ranging from 12.1 to 34.7μm were investigated by the 90° V-bending tests at 150 °C. The results show that the neutral layer tends to shift to outer region of the sheets and the coefficient of neutral layer value (k-value) increases with the increasing grain size. This phenomenon is mainly owing to the enhanced asymmetry between the outer tension region and inner compression region with the increase of grain size. Twinning dominates the deformation in inner region while slips dominate the deformation in outer region.展开更多
In this study, the development of a convective boundary layer （CBL） in the Badaujilin region was investigated by comparing the observation data of two cases. A deep neutral layer capped a CBL that occurred on 30 Aug...In this study, the development of a convective boundary layer （CBL） in the Badaujilin region was investigated by comparing the observation data of two cases. A deep neutral layer capped a CBL that occurred on 30 August 2009. This case was divided into five sublayers from the surface to higher atmospheric elevations： surface layer, mixed layer, inversion layer, neutral layer, and sub-inversion layer. The development process of the CBL was divided into three stages： S1, S2, and S3. This case was quite different from the development of the three-layer CBL observed on 31 August 2009 because the mixed layer of the five-layer CBL （CBL5） eroded the neutral layer during S2. The specific initial structure of the CBL5 was correlated to the synoptic background of atmosphere during nighttime. The three-stage development process of the CBL5 was confirmed by six simulations using National Center for Atmospheric Research （USA） large-eddy simulation （NCAR-LES）, and some of its characteristics are presented in detail.展开更多
The neutral layer(NL)strategy is a key technique for improving the bendability of flexible electronic devices.In this study,by considering a three-layer structure as an example,the results obtained by finite element a...The neutral layer(NL)strategy is a key technique for improving the bendability of flexible electronic devices.In this study,by considering a three-layer structure as an example,the results obtained by finite element analysis(FEA)showed that the NL gradually moved to the top surface of the film as the film thickness and Young’s modulus increased,which are similar to the results produced by theoretical calculations.Subsequently,we optimized the thickness of a single NL structure and the failure bending radius of an indium tin oxide(ITO)electrode was reduced by 50%after optimization.In order to address the problems that affect the design of a single NL,we used optical clear adhesive(OCA)to generate multiple NLs.The FEA method was again applied to the structure and the results showed that decreasing the elastic modulus of the OCA and film thickness could reduce the maximum strain in the film.Finally,the effects of the OCA parameters on the protection of a multiple-layer ITO electrode structure were verified in bending experiments,which showed that the strain on ITO could be reduced from 5.6%to almost 0 in the two-electrode structure.The proposed strategies for designing single and multiple NLs can provide some guidance to facilitate optimizing the electronic infrastructure of flexible devices.展开更多
Based on Zeevaert＇s method, a theoretical formula was developed to calculate the negative skin friction of pile in layered soil. For practical purpose,a cut-and-try method was proposed to determine neutral point. Cas...Based on Zeevaert＇s method, a theoretical formula was developed to calculate the negative skin friction of pile in layered soil. For practical purpose,a cut-and-try method was proposed to determine neutral point. Case studies indicate that the total calculated negative skin friction was in agreement with the measured one, which verifies the feasibility and practicability of theoretical formula. Furthermore, the methods for calculating efficiency factor of drag load and settlement were also given.展开更多
Electrical characteristics of a nozzle-attached meso-scale premixed methane-air flame under low-frequency AC （0-4300 V, 0-500 Hz） and DC （0-3300 V） electric fields were studied. I-V curves were measured under diff...Electrical characteristics of a nozzle-attached meso-scale premixed methane-air flame under low-frequency AC （0-4300 V, 0-500 Hz） and DC （0-3300 V） electric fields were studied. I-V curves were measured under different experimental conditions to estimate the magnitude of the total current 10^0-10^2μA, the electron density 10^15-10^16 m^-3 and further the power dissipation 〈 0.7 W in the reaction zone. At the same time, the meso-scale premixed flame conductivity 10^-4-10^-3 Ω^-1. m^-1 as a function of voltage and frequency was experimentally obtained and was believed to represent a useful order-of magnitude estimate. Moreover, the influence of the collision sheath relating to Debye length （31-98 μm） and the contamination layer of an active electrode on measurements was discussed, based on the combination of simulation and theoretical analysis. As a result, the electrode sheath dimension was evaluated to less than 0.5 mm, which indicated a complex effect of the collision sheath on the current measurements. The surface contamination effect of an active electrode was further analyzed using the SEM imaging method, which showed elements immigration during the contamination layer formation process.展开更多
The study on the global instability of a Stokes layer, which is a typical unsteady flow, is usually a paradigm for understanding the instability and transition of unsteady flows. Previous studies suggest that the neut...The study on the global instability of a Stokes layer, which is a typical unsteady flow, is usually a paradigm for understanding the instability and transition of unsteady flows. Previous studies suggest that the neutral curve of the global instability obtained by the Floquet theory is only mapped out in a limited range of wave numbers （0.2 ≤ a ≤ 0.5）. In this paper, the global instability is investigated with numerical simulations for all wave numbers. It is revealed that the peak of the disturbances displays irregularity rather than the periodic evolution while the wave number is beyond the above range. A ＂neutral point＂ is redefined, and a neutral curve of the global instability is presented for the whole wave numbers with this new definition. This work provides a deeper understanding of the global instability of unsteady flows.展开更多
GaN基高电子迁移率晶体管(High Electron Mobility Transistor,HEMT)因其在高频、高功率和高温应用中的优异性能而受到广泛关注。金属-绝缘体-半导体高电子迁移率晶体管(Metal-Insulating-Semiconductor High Electron Mobility Transis...GaN基高电子迁移率晶体管(High Electron Mobility Transistor,HEMT)因其在高频、高功率和高温应用中的优异性能而受到广泛关注。金属-绝缘体-半导体高电子迁移率晶体管(Metal-Insulating-Semiconductor High Electron Mobility Transistor,MIS-HEMT)通过引入栅极绝缘层来改善器件的性能,特别是减少栅极泄漏电流和提高击穿电压。然而使用凹栅结构的MIS-HEMT在实现阈值电压(Vth)均匀性上存在挑战。使用Silvaco TCAD软件对具有三重势垒层的凹栅结构增强型AlGaN/GaN MIS-HEMT进行了数值研究,与传统肖特基栅极HEMT相比,三势垒层增强型AlGaN/GaN MIS-HEMT采用压电中和技术(Piezoelectric Neutralization Technology,PNT),实现了高阈值电压均匀性。为了使器件获得更好的直流特性,研究了改变缓冲层和PNT层Al质量分数对器件的影响。此外,还详细分析了器件尺寸缩放(例如栅漏距离Lgd和栅长Lg)对直流特性的依赖性。优化后的MIS-HEMT的Vth为0.8 V,输出电流为0.43 A/mm,饱和漏电流为1.99 A/mm,最大跨导(gmmax)为0.14 S/mm,比导通电阻(Ron,sp)为1.64 mΩ·cm^(2)。研究结果对于三势垒层AlGaN/GaN MIS-HEMT结构的设计或优化具有重要的指导意义。展开更多
基金supported by the National Natural Science Foundation of China(No.52371004,52201144,52071040,U20A20230).
文摘Bending is a crucial operation in the sheet forming of Mg alloys for automotive and aerospace applications.In this work,three kinds of sheets from an AZ31 rolled plate,sheared at 0°(S0),45°(S45),and 90°(S90)to the rolling direction,were subjected to three-point bending tests.In-situ digital image correlation(DIC)system was employed to capture the strain fields.Electron backscatter diffraction(EBSD)was used to examine the through-thickness microstructures.A crystal plasticity finite element method(CPFEM)incorporating twinning and slip mechanisms was developed to simulate the bending deformation.The texture effect on the neutral layer shift and twinning behaviors was systematically investigated in terms of both experiments and simulations.The results show that CPFEM effectively simulated the texturedependent shifting behaviors of neutral layer and the associated twinning behaviors.Particularly,the spatial distributions of neutral layer across the entire bent plates were captured by both DIC and CPFEM.Additionally,some unusual twinning behaviors were analyzed in depth,such as the{10-12}-{10-12}secondary twins in S90,localized twin bands in S0,and the twin traces difference in S45.These findings reveal a close relationship between the neutral layer shift and twinning activity induced by the initial texture and strain levels.This work provides valuable insights into the bending deformation mechanisms of Mg alloys and has important implications for improving their formability and controlling springback.
基金Project(CSTC2012GGB50003)supported by Scientific and Technological Project of Chongqing Science and Technology Commission,ChinaProject(CDJZR13130081)supported by the Fundamental Research Funds for the Central Universities,China
文摘The effects of grain size on the shift of neutral layer of AZ31 magnesium alloy sheets with different grain sizes ranging from 12.1 to 34.7μm were investigated by the 90° V-bending tests at 150 °C. The results show that the neutral layer tends to shift to outer region of the sheets and the coefficient of neutral layer value (k-value) increases with the increasing grain size. This phenomenon is mainly owing to the enhanced asymmetry between the outer tension region and inner compression region with the increase of grain size. Twinning dominates the deformation in inner region while slips dominate the deformation in outer region.
基金funded by National Basic Research Program of China (Grant Nos. 2009CB421402 and 2010CB950503)National Natural Science Foundation of China (GrantNo. 40975007)
文摘In this study, the development of a convective boundary layer （CBL） in the Badaujilin region was investigated by comparing the observation data of two cases. A deep neutral layer capped a CBL that occurred on 30 August 2009. This case was divided into five sublayers from the surface to higher atmospheric elevations： surface layer, mixed layer, inversion layer, neutral layer, and sub-inversion layer. The development process of the CBL was divided into three stages： S1, S2, and S3. This case was quite different from the development of the three-layer CBL observed on 31 August 2009 because the mixed layer of the five-layer CBL （CBL5） eroded the neutral layer during S2. The specific initial structure of the CBL5 was correlated to the synoptic background of atmosphere during nighttime. The three-stage development process of the CBL5 was confirmed by six simulations using National Center for Atmospheric Research （USA） large-eddy simulation （NCAR-LES）, and some of its characteristics are presented in detail.
基金supported by the Natural Science Foundation,China(Grant No.52175512)Huawei Technologies Co.,Ltd.
文摘The neutral layer(NL)strategy is a key technique for improving the bendability of flexible electronic devices.In this study,by considering a three-layer structure as an example,the results obtained by finite element analysis(FEA)showed that the NL gradually moved to the top surface of the film as the film thickness and Young’s modulus increased,which are similar to the results produced by theoretical calculations.Subsequently,we optimized the thickness of a single NL structure and the failure bending radius of an indium tin oxide(ITO)electrode was reduced by 50%after optimization.In order to address the problems that affect the design of a single NL,we used optical clear adhesive(OCA)to generate multiple NLs.The FEA method was again applied to the structure and the results showed that decreasing the elastic modulus of the OCA and film thickness could reduce the maximum strain in the film.Finally,the effects of the OCA parameters on the protection of a multiple-layer ITO electrode structure were verified in bending experiments,which showed that the strain on ITO could be reduced from 5.6%to almost 0 in the two-electrode structure.The proposed strategies for designing single and multiple NLs can provide some guidance to facilitate optimizing the electronic infrastructure of flexible devices.
基金Shandong Provincial Education Department Fund (No.J09LE05)
文摘Based on Zeevaert＇s method, a theoretical formula was developed to calculate the negative skin friction of pile in layered soil. For practical purpose,a cut-and-try method was proposed to determine neutral point. Case studies indicate that the total calculated negative skin friction was in agreement with the measured one, which verifies the feasibility and practicability of theoretical formula. Furthermore, the methods for calculating efficiency factor of drag load and settlement were also given.
基金supported by National Natural Science Foundation of China(No.51376021)the Fundamental Research Fund for Major Universities(No.2013JBM079)
文摘Electrical characteristics of a nozzle-attached meso-scale premixed methane-air flame under low-frequency AC （0-4300 V, 0-500 Hz） and DC （0-3300 V） electric fields were studied. I-V curves were measured under different experimental conditions to estimate the magnitude of the total current 10^0-10^2μA, the electron density 10^15-10^16 m^-3 and further the power dissipation 〈 0.7 W in the reaction zone. At the same time, the meso-scale premixed flame conductivity 10^-4-10^-3 Ω^-1. m^-1 as a function of voltage and frequency was experimentally obtained and was believed to represent a useful order-of magnitude estimate. Moreover, the influence of the collision sheath relating to Debye length （31-98 μm） and the contamination layer of an active electrode on measurements was discussed, based on the combination of simulation and theoretical analysis. As a result, the electrode sheath dimension was evaluated to less than 0.5 mm, which indicated a complex effect of the collision sheath on the current measurements. The surface contamination effect of an active electrode was further analyzed using the SEM imaging method, which showed elements immigration during the contamination layer formation process.
基金Project supported by the National Natural Science Foundation of China(No.11202147)the Specialized Research Fund for the Doctoral Program of Higher Education of China(No.20120032120007)
文摘The study on the global instability of a Stokes layer, which is a typical unsteady flow, is usually a paradigm for understanding the instability and transition of unsteady flows. Previous studies suggest that the neutral curve of the global instability obtained by the Floquet theory is only mapped out in a limited range of wave numbers （0.2 ≤ a ≤ 0.5）. In this paper, the global instability is investigated with numerical simulations for all wave numbers. It is revealed that the peak of the disturbances displays irregularity rather than the periodic evolution while the wave number is beyond the above range. A ＂neutral point＂ is redefined, and a neutral curve of the global instability is presented for the whole wave numbers with this new definition. This work provides a deeper understanding of the global instability of unsteady flows.
